Power integrated circuit with autonomous limit checking of ADC channel measurements

ABSTRACT

A power loss protection integrated circuit includes a current switch circuit portion (eFuse) and an autonomous limit checking circuit. The limit checking circuit includes an input analog multiplexer, an ADC, a plurality of capture registers, a state machine, and a flag output terminal. For each capture register, the limit checking circuit further includes an associated lower limit register and an associated upper limit register. The state machine controls the multiplexer and the capture registers so the ADC digitizes voltages on various nodes to the monitored, and stores the results into corresponding capture registers. In integrated circuit has circuitry that allows both a high voltage as well as a high current to be monitored. The value in a capture register is compared to upper and lower limit values. If any capture value is determined to be outside the limits, then a digital flag signal is asserted onto the flag output terminal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.15/201,233 entitled “Power Integrated Circuit With Autonomous LimitChecking Of ADC Channel Measurements,” filed on Jul. 1, 2016, now U.S.Pat. No. 9,721,742. Application Ser. No. 15/201,233 in turn claims thebenefit under 35 U.S.C. §119 of each of the following U.S. ProvisionalApplications: 1) U.S. Provisional Application Ser. No. 62/354,732,entitled “Power Integrated Circuit With Autonomous Limit Checking Of ADCChannel Measurements,” filed Jun. 25, 2016; 2) U.S. ProvisionalApplication Ser. No. 62/354,733, entitled “Power Loss ProtectionIntegrated Circuit,” filed Jun. 25, 2016; 3) U.S. ProvisionalApplication Ser. No. 62/354,734, entitled “Autonomous Capacitor HealthCheck In A Power Loss Protection Integrated Circuit,” filed Jun. 25,2016; 4) U.S. Provisional Application Ser. No. 62/354,738, entitled“Programmable Voltage Setting With Open And Short Circuit Protection,”filed Jun. 25, 2016; 5) U.S. Provisional Application Ser. No.62/354,740, entitled “Optimized Gate Driver For Low Voltage Power LossProtection System,” filed Jun. 25, 2016; 6) U.S. Provisional ApplicationSer. No. 62/354,741, entitled “Fast Settlement Of Supplement ConverterFor Power Loss Protection System,” filed Jun. 25, 2016; and 7) U.S.Provisional Application Ser. No. 62/357,739, entitled “Gate Driver ToDecrease EMI With Shorter Dead-Time,” filed Jul. 1, 2016. The entiresubject matter of the aforementioned patent documents is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates generally to power loss protectionsystems, and to related circuits and methods.

BACKGROUND INFORMATION

Capacitors and/or batteries are used to store energy in power lossprotection systems. FIG. 1 (Prior Art) illustrates one example of apower loss protection system 200 involving a so-called “eFuse” 201, aswitch component 202, enable trip point resistors 203 and 204, a boostconverter circuit 205, a bank of capacitors and/or batteries 206, and avoltage regulator 207. This circuitry is soldered onto a printed circuitboard. During normal operation of the system, the boost converter 201receives some of the power from the eFuse output and uses that to chargethe capacitors and/or batteries 202. If VIN at input 203 is thensuddenly lost, for example due to a condition like a power brown outcondition or a power cord disconnection event, then the eFuse turns offimmediately and the fault signal is asserted. The fault signal isreceived by the second switch SW2 202. The second switch SW2 202responds by turning on so that power can then be supplied onto node 208from the capacitors and/or batteries 202 in the place of the lost power.The linear voltage regulator 207 reduces the capacitor voltage down tothe supply voltage “VSYS” required by the device being powered. Therequired system power and supply voltage VSYS can therefore bemaintained for long enough to allow data that is stored in the device involatile memory to be transferred into non-volatile storage beforesystem power is lost altogether. This kind of prior art power lossprotection system is used to power devices where high reliability isrequired, such as in computers like laptops and servers. If capacitorsare used for block 206, then the capacitance of the capacitors must belarge enough that the energy stored in the capacitors can sustain systempower long enough to allow the device to complete a safe power-offsequence involving backup of configuration information and status aftera loss of VIN is detected. The capacitors are sized accordingly toaccount for anticipated system operation and the needs of the devicebeing powered.

SUMMARY

In a power loss protection system as described above in the backgroundinformation section, in which capacitors and not batteries are used forblock 202, one or more of the capacitors of block 202 may degrade forvarious reasons. In other systems of the very same type, thecorresponding capacitors do not degrade. Due to the degradation, thecapacitor or capacitors may have a smaller capacitance. When the powerloss protection system is needed in a power loss event, the remainder ofthe circuit works but there may be inadequate energy stored due to thelower than required capacitance where the backup energy is stored.

In a first novel aspect, a power loss protection integrated circuitincludes a current switch circuit portion (that can perform the eFusefunction) and an autonomous limit checking circuit. The autonomous limitchecking circuit includes an input analog multiplexer, anAnalog-to-Digital Converter (ADC), a plurality of capture registers, adigital state machine, and a flag output terminal. For each captureregister, the autonomous limit checking circuit further includes anassociated lower limit register and an associated upper limit register.The digital state machine controls the input analog multiplexer and thecapture registers so the ADC digitizes a voltage on a first of aplurality of nodes to be monitored. The digitized value from the ADC isthen stored under control of the digital state machine in acorresponding first one of the capture registers. After this captureevent, the digital state machine controls the input analog multiplexerand the capture registers so that the ADC digitizes a voltage on asecond one of a plurality of nodes, and the resulting second digitizedvalue is then stored in a corresponding second one of the captureregisters. One by one, the voltages on the various nodes to be monitoredare digitized, and the resulting digital capture values are stored oneby one into the corresponding ones of the capture registers. The capturevalue in each capture register is compared to a upper limit value storedin the associated upper limit register and is also compared to a lowerlimit value stored in the associated lower limit register. Thesecomparisons may be masked. A digital value indicative of a result of thecomparisons is obtained. In one example, the digital value is true ifthe capture value is smaller than the lower limit value or is greaterthan the upper limit value. If any of these digital values for any ofthe capture registers is asserted, then a flag signal indicative of thiscondition is output onto the flag output terminal. This capturing andcomparing operation of the limit checking circuit is “autonomous” fromany associated digital processor or microcontroller that might becoupled to the integrated circuit in that once it is set up, it operatesautonomously. The power loss protection integrated circuit is a powerintegrated circuit device that includes rugged power transistors andcircuitry in its current switch portion that are not found in typicalpurely digital logic integrated circuits. In one example, a node voltagethat is monitored can have a voltage that is substantially higher thanthe supply voltage that powers the digital logic circuitry on the powerloss protection integrated circuit. For example, a twenty volt nodevoltage may be monitored and a capture value obtained that is indicativeof the magnitude of this voltage, whereas the digital logic circuitry ispowered from a VDD supply voltage of five volts or less. In addition,the power loss protection includes a current sense circuitry that allowsthe monitoring of a large through-current flowing through the currentswitch circuit. This through-current (for example, a constant twoamperes) is much larger than the typical small currents that wouldordinarily be monitored by a discrete ADC. The ADC is capable ofmonitoring the magnitude of this large through-current. Advantageously,the power loss protection integrated circuit includes no processor thatfetches, decodes and executes any instructions. The limit checkingcircuit can operate autonomously of any such processor. Associated witheach limit register is a corresponding register of non-volatile memory,such as FLASH. These non-volatile registers are preloaded with data atthe time of integrated circuit device manufacture by the integratedcircuit manufacturer and supplier. The very same type of power lossprotection integrated circuit that is used in a larger circuit involvinga processor can also be used in another customer's system that has noprocessor. Upon power up and/or at another suitable time, the pre-loadedcontent of each non-volatile register is automatically loaded into itsassociated limit register. In one case, the flag output terminal isusable to supply an interrupt signal to a processor, such as amicrocontroller of the device being powered. In another case, the flagoutput terminal is usable to drive a LED that gives a visible indicationof a detected limit situation in a case where the user's system mighthave no processor.

In a second novel aspect, a single power loss protection integratedcircuit includes a current switch circuit (that can perform the eFusefunction), an input voltage terminal (VIN), an output voltage terminal(VOUT), a buck/boost controller, and a storage capacitor terminal (STR).The buck/boost controller is adapted to work, along with discretecomponents external to the integrated circuit such as a discreteinductor, as either a buck converter in a buck mode of operation, or asa boost converter in a boost mode of operation. The same externalinductor is used as the inductor in both modes. In the boost mode, thebuck/boost controller drives a switch terminal so that the buck/boostcontroller in combination with the external inductor receives arelatively low voltage as output by the current switch circuit (theeFuse) and outputs a relatively high voltage onto the storage capacitorterminal (STR) thereby charging any external capacitor coupled to thisterminal. In the buck mode, the buck/boost controller receives therelatively high voltage from the storage capacitor terminal (STR) andbucks it down to the relatively low voltage that is required at theoutput voltage terminal VOUT by the device to be powered. The currentswitch circuit in performing its eFuse function can pass athrough-current of up to a maximum current from the input voltageterminal VIN to the output voltage terminal VOUT, where this maximuminput current is a large current of at least two amperes. Thethrough-current is large as compared to currents that typically passthrough digital logic circuits. The current switch circuit outputs adigital undervoltage signal and a digital high current signal. Thedigital undervoltage signal (UV), if asserted, indicates that the inputvoltage on the VIN terminal is lower than a predetermined voltage. Thedigital high current signal (HC), if asserted, indicates that thethrough-current is greater than a predetermined current. If the digitalundervoltage signal UV is asserted, then the buck/boost converter ismade to begin operating in the buck mode. If the digital high currentsignal HC is asserted, then the buck/boost converter is prevented fromoperating in the boost mode. Advantageously, the intercommunication ofthe necessary signals between the current switch circuit portion of theintegrated circuit and the buck/boost controller portion of theintegrated circuit to accomplish these conditional disabling functionsare on the integrated circuit. This allows the signaling to be fast, andallows the signals to be lower power signals because the environmenton-chip is relatively free of potential noise problems that otherwisemight be present were the intercommunication to have to pass acrossconductors on a printed circuit board. Potential board levelcomplexities and noise problems are avoided. The speed and efficiency ofthe intercommunication allows the size of the storage capacitor bank fora given application to be made smaller and less expensive because thebuck/boost converter is controlled faster in a fault condition ascompared to a prior art situation as illustrated in FIG. 1 of thebackground information section.

In a third novel aspect, a power loss protection integrated circuitincludes a storage capacitor terminal (STR), a novel autonomouscapacitor health check circuit, and a capacitor fault terminal (CF). Thenovel autonomous capacitor health check circuit is “autonomous” in thatit can be set up to operate autonomously of any instruction fetching andexecuting processor as described above in connection with the autonomouslimit checking circuit. The capacitor health check circuit performs aperiodic capacitor check operation by sinking a predetermined currentfrom the storage capacitor terminal for a predetermined time duration.During the predetermined time duration it detects whether the voltage onthe storage capacitor terminal STR drops below a predetermined thresholdvoltage. If the voltage on the storage capacitor terminal STR isdetected to have dropped below the predetermined threshold voltage, thena sequential logic element in the capacitor health check circuit is set,and this in turn causes a digital logic capacitor fault signal CF to beasserted onto the capacitor fault terminal CF. Immediately following thecapacitor health check operation, a buck/boost controller portion of theintegrated circuit may again operate in a boost mode so that a chargingvoltage is again driven onto the storage capacitor terminal (STR),thereby recharging the external storage capacitor back up to itsinitial, fully charged, voltage. Such a capacitor health check operationis carried out periodically by the capacitor health check circuitrywithout having to be repeatedly prompted to do such a check by anydigital processor. In one example, the power loss protection integratedcircuit includes a buck/boost controller as discussed above. Thecapacitor health check circuit outputs a digital control signal thatdisables the controller from operating when a capacitor health checkoperation is occurring. In one example, the predetermined time durationactually involves a first time portion and a second time portion. Duringthe first time portion, a first predetermined and fixed current is“sinked” from (drawn from to a current sink) the STR terminal. Duringthe second time portion, a second predetermined and fixed current issinked from the STR terminal. After the capacitor health check operationis completed, the overall power loss protection integrated circuitautomatically resumes operating in its normal mode. The first timeportion of the test is tailored to detect a first failure characteristicof a degraded capacitor, and the second time portion of the test istailored to detect a second failure characteristic of a degradedcapacitor. In one example, an external processor can detect whether thesequential logic element of the capacitor health check circuit has beenset or not. The external processor does this by reading a register ofthe integrated circuit through a serial digital bus interface. If a bitin this register is read as having been set, this indicates to theprocessor that the sequential logic element of the capacitor healthcheck circuit was set. The reading of the register causes the sequentiallogic element to be reset. The very same type of power loss protectionintegrated circuit is usable both with a digital processor that can readthe register, and without such a processor. In one example, after acertain amount of time has elapsed, the sequential logic element of thehealth check circuit is automatically reset regardless of whether theregister was read or not. In one example, there are various modes forhow the capacitor fault signal is communicated out of the integratedcircuit, and how the sequential logic element is reset. There is adefault mode. The mode used at a give time can be changed by an externalprocessor by writing to a control register on the integrated circuit viathe serial bus interface.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequentlyit is appreciated that the summary is illustrative only. Still othermethods, and structures and details are set forth in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a diagram of an example of a power loss protection system.

FIG. 2A is a first part of a larger diagram of a system involving anovel power loss protection integrated circuit.

FIG. 2B is a second part of a larger diagram of a system involving anovel power loss protection integrated circuit. FIG. 2A and FIG. 2Btogether form the larger diagram of FIG. 2.

FIG. 3 is a more detailed diagram of the current switch circuit 57 ofFIG. 2.

FIG. 4 is a more detailed diagram of the health monitor circuit 59 ofFIG. 2.

FIG. 5 is a more detailed diagram of a compare-and-mask circuit 95within the health monitor circuit 59 of FIG. 4.

FIG. 6 is a more detailed diagram of the input analog multiplexercircuit 110 of the health monitor circuit 59 of FIG. 4.

FIG. 7 is a more detailed diagram of the DAC and compare circuit 118 ofthe health monitor circuit 59 of FIG. 4.

FIG. 8 is a state diagram for the capacitor health monitor state machine117 of FIG. 4.

FIG. 9 is a table that indicates, for each 4-bit TSET[3:0] value, thecorresponding time period throughout which the ten milliampere currentsink 115 of the health monitor circuit 114 of FIG. 4 is enabled during acapacitor health check operation.

FIG. 10 is a timeline diagram that illustrates one capacitor healthcheck operation as carried out by the capacitor health check circuit 114of FIG. 4.

FIG. 11 is a more detailed diagram of the switching converter controlcircuit 61 of FIG. 1.

FIG. 12 is a more detailed diagram of the voltage reference circuit 62of FIG. 2.

FIG. 13 is a more detailed block diagram of the I2C interface anddigital register control and digital state machine block 58 of FIG. 2.

DETAILED DESCRIPTION

FIG. 2 is a diagram of a system 1 that involves a device to be powered 2and an AC-to-DC adapter 3. The device 2 in this particular example is aSolid State Drive (SSD) device. It includes a solid state device system4, a Power Management Unit (PMU) integrated circuit 5, a power lossprotection integrated circuit 6, a bank of series-connected batteries7-10, a bank of parallel-connected capacitors 11, a socket portion 12and 13 of a connector, a DC-to-DC converter and battery charger 14, andother discrete components 15-29. The solid state drive system 4includes, among other components not shown, a solid state memory portion33 and a microcontroller integrated circuit 34. The AC-to-DC adapter 3can be connected to the SSD device 2 by plugging its plug portion 30 and31 into the socket portion 12 and 13 of the SSD device, and the AC-to-DCadapter 3 can be disconnected from the SSD device by unplugging it fromthe socket portion. The AC-to-DC adapter 3 is coupled to receive powerfrom a 120 volt RMS VAC 60 Hz supply voltage source 32 such as, forexample, a standard residential wall socket. The SSD device 2 furtherincludes a printed circuit board (not shown) disposed within anenclosure (not shown). The integrated circuits and other components ofthe SSD device 2 are mounted to the PCB and are contained in theenclosure.

Power loss protection integrated circuit 6 includes a VIN terminal 35,an EN terminal 36, a CSS terminal 37, an ISET terminal 38, severalanalog input terminals 39-43, a STR terminal 44, a BSET terminal 45, aground terminal GND 46, a CCOMP terminal 47, a VOUT terminal 63, an SCLterminal 48, a SDA terminal 49, a flag output terminal 50, a capacitorflat terminal CF 51, an HSB terminal 52, a SW terminal 53, a PGNDterminal 54, an FB terminal 55, a REF terminal 56, a current switchcircuit 57, an I2C interface and digital register control and digitalstate machine circuit 58, a health monitor circuit 59, an on-chiptemperature sensor 60, a buck/boost switching converter control circuit61, and a reference voltage circuit 62.

The current switch circuit 57 is also called an eFuse circuit. Thecurrent switch circuit 57 can couple the VIN terminal 35 to the VOUTterminal 63 such that current can freely flow from the VIN terminal 35,through the current switch circuit 57, and to the VOUT terminal 63. Whenthe current switch circuit 57 is ON in this way, it only introduces a 15milliohm resistance in that current path. The current switch circuit 57monitors the voltage on the VIN terminal. If the voltage on the VOUTterminal 63 is greater than the voltage on the VIN terminal 35, then theswitch circuit 57 asserts the VOUT>VIN digital signal on conductor 86 toa digital logic high, otherwise the VOUT>VIN digital signal is a digitallogic low. If the voltage on the VIN terminal is below an undervoltagevalue set by resistors 15 and 16, then the current switch circuit 57 isOFF such that the VIN terminal is not coupled to the VOUT terminalthrough the switch circuit 57. If the current switch circuit 57 detectsthe “UV” undervoltage condition, then it asserts the UV digital signalon conductor 64 to a digital logic high, otherwise the UV digital signalis a digital logic low. If the voltage on the VIN terminal is above aprogrammable overvoltage value, then the current switch circuit 57 isOFF such that the VIN terminal is not coupled to the VOUT terminalthrough the switch circuit 57. If the current switch 57 detects the “OV”overvoltage condition, then it asserts the OV digital signal on the OVconductor 85.

In addition to sensing voltages, the current switch circuit 57 alsosenses the magnitude of current flowing through the current switchbetween the VIN terminal and the VOUT terminal. If the current is belowa predetermined high current value (A_(HC)), and if the current switch57 is to be ON as determined by the voltage on the VIN terminal, thenthe current switch is fully ON (to have a resistance of 15 milliohms orless). If, however, the current is detected to reach the high currentvalue (A_(HC)), then the current switch circuit begins to regulate thethrough-current so that the through-current remains at the high currentvalue amount A_(HC) but does not exceed A_(HC). The current switch 57does this by controlling the gate voltages on a pair of series fieldeffect transistors through which the through-current flows. Increasingthe drain-to-source resistance R_(DS) of these field effect transistorsallows the flow of current to be maintained at the A_(HC) amount. If,however, the R_(DS) across the transistors becomes too high, or if thevoltage on the VOUT terminal decreases too much, then the field effecttransistors are not linearly regulated by controlling their R_(DS)resistances, but rather the field effect transistors are turned on andoff repeatedly with a duty cycle. The duty cycle is regulated in anattempt to limit the power dropped in the current switch circuit 57. Inthis way, the current switch circuit 57 serves a function of limitingthe magnitude of a possible large inrush current (inrush power) thatmight otherwise flow into the system when the SSD device is initiallyplugged into the AC-to-DC adapter 3 when the storage capacitors 11 arefully discharged and when the COUT capacitor 21 is fully discharged. Inthe present example, the inrush current limit set by the resistance ofresistor RSET 18 is a current (for example, two amperes) that is largerthan a typical digital logic or analog signaling input terminal oroutput terminal could handle.

The CSS capacitor 17 slows down the start up slew rate of the currentswitch circuit 57, thereby providing a “soft start” operation. The boarddesigner can select the capacitance value of the CSS capacitor to tailorthe start up slew rate as desired. If left open, the startup slew ratedefaults to one millivolt per microsecond. The high current value(A_(HC)) is set by setting the resistance value of resistor RSET 18. Thehigh current value A_(HC) is roughly equal to one volt divided by theRSET value in ohms. If the current switch circuit 57 detects the “HC”high current condition, then it asserts the HC digital signal onconductor 65 to digital logic high, otherwise the HC digital signal is adigital logic low. The current switch circuit 57 includes a currentsensor/mirror that provides a small auxiliary current flow whosemagnitude is proportional to the magnitude of the main current flowthrough the current switch circuit 57 from the VIN terminal to the VOUTterminal. This small mirrored auxiliary current is converted into avoltage signal by making the current flow across the RSET resistor 18.The resulting voltage signal, whose magnitude is proportional to thecurrent flow through the switch circuit 57, is output from the currentswitch circuit 57 via the switch current (SC) conductor 66. The voltagesignal on the switch current SC conductor 66 is indicative of themagnitude of the current flowing through the current switch 57.

The buck/boost switching converter control circuit 61, together withexternal components 20, 21 and 23-27 is operable as a buck switchingconverter or as a boost switching converter. When it is operating in aboost mode, the converter receives a relatively low voltage from theVOUT terminal, and outputs a boosted up relatively high voltage onto theSTR terminal 44. In one example, the voltage on the VOUT terminal is 3.3volts DC, and the voltage that the converter drives onto the STRterminal 44 is 36 volts DC. This relatively high voltage serves tocharge the capacitor bank 11 capacitors up to 36 volts. The magnitude ofthis charging voltage is set by the value of the RBSET resistor 19. Whenthe converter is operating in a buck mode, the converter receives arelatively high voltage from the STR terminal 44, and outputs a buckeddown relatively low voltage onto the VOUT terminal 63. In one example,the voltage on the STR terminal 44 is 36 volts (as set by the RBSETresistor), and the voltage that the converter drives onto the VOUTterminal is 3.3 volts DC. The buck/boost switching converter controlcircuit 61 has an active high boost disable digital signal input leadBOOST_DIS1 67 and another active high boost disable digital input leadBOOST_DIS2 68. If a digital logic high signal is present on either ofthese inputs, then the converter is prevented (disabled) from operatingin the boost mode. The buck/boost switching converter control circuit 58also has an active high digital signal input lead BUCK ON 69. If adigital logic high signal is present on this input 69, then theconverter is made to start operating in the buck mode.

The health monitor circuit 59 includes an eight-channel sigma-deltaAnalog-to-Digital Converter (ADC), a set of compare-and-mask circuits,and a digital state machine. The health monitor circuit 59 autonomouslymonitors the voltages on eight input conductors 66, 69, and 71-76 (8channels). If any one of these voltages is detected to be below acorresponding lower voltage limit or is detected to be above acorresponding upper voltage limit, then this undervoltage or overvoltagecondition is latched into a latch of the detecting compare-and-maskcircuit, and the voltage on flag terminal 50 is pulled down to groundpotential. The voltage on the open-drain flag terminal 50 is otherwisenot pulled down, but rather is pulled up to the VDD by external pullupresistor 22. The low voltage (ground potential) on flag terminal 50 andconductor 77 constitutes an interrupt signal 78. This active lowinterrupt signal 78 is supplied via conductor 77 onto the active lowinterrupt input terminal 79 of microcontroller 34. The low interruptsignal therefore interrupts the microcontroller 34. The microcontroller34 can respond to the interrupt, as further explained below, byaccessing the power loss protection integrated circuit 6 via thetwo-wire I2C bus 80. The two conductors SDL and SDA are the twoconductors of the I2C bus. The values of the lower voltage limit and theupper voltage limit for each of the eight channels is user programmable(changeable via the microcontroller 34 under software control) via theI2C interface of terminals 48 and 49. In the present example, themeasurable voltage range on conductor 66 corresponds to a measuredthrough-current flowing through the current switch 57 in the range offrom zero amperes to six amperes. In the present example, the measurablevoltage range on conductor 69 corresponds to a measured voltage on theVIN terminal in the range of from zero volts to twenty volts. In thepresent example, the measurable voltage range on conductor 71corresponds to a measured storage capacitor voltage on the STR terminalin the range of from zero volts to thirty-six volts. In the presentexample, the measurable voltage range on conductor 72 corresponds to ameasured on-chip temperature in the range of from minus forty degreesCelsius to plus one hundred and fifty degrees Celsius.

The health monitor circuit 59 also includes a capacitor health checkcircuit. The capacitor health check circuit includes a digital statemachine. If the power loss protection integrated circuit 6 is notoperating in the normal mode as indicated by the active high NORMAL_MODEdigital signal on conductor 88, then the capacitor health check circuitis disabled. If, however, the power loss protection integrated circuit 6has been operating in the normal mode for a least four minutes, at theconclusion of the four minute period the state machine disables theboost converter and enables a ten milliampere current source. The tenmilliampere current source sinks current from the STR terminal 44. Atthe end of a time period determined by the programmable value TSET[3:0],the state machine disables the ten milliampere current source andenables a fifty milliampere current source that sinks current from theSTR terminal 44. The fifty milliampere current source remains enabledfor a period of time determined by the value TSET[3:0]. In one example,this time period is one tenth the period of time the ten milliamperecurrent source was enabled. If at any time during the period of timewhen either of the two sinking current sources is enabled the voltage onthe STR terminal 44 falls below a programmable voltage, then a latch isset. The programmable voltage is determined by the user programmablevalue THR[3:0]. The setting of the latch causes the voltage on thecapacitor fault terminal CF 51 to be pulled down to ground potential.This is an indication of a capacitor fault condition. This active lowfault signal may, for example, be supplied onto a second interrupt inputterminal 87. In addition, the LED 28 is on during the time when thencapacitor fault signal is asserted low.

The I2C interface and digital register control and digital state machinecircuit 58 is a digital block that includes an I2C serial bus interfacecircuit and a digital state machine circuit. There are various digitalregisters disposed in various places across the integrated circuit. Thedigital outputs of various ones of the bits of these registers arecoupled to various circuits in the integrated circuit so that the storeddigital values will control and affect operation of the circuitry. Otherselected bits of the registers are used to capture the digital states ofcorresponding nodes in the circuitry. The I2C interface is usable toread and to write to any selected one of these registers via the DATAconductors 81, the enable conductors 82, the R/W conductor 83 and thestrobe conductor 84. The DATA conductor 81, the R/W conductor 83, andthe strobe conductor 84 extend to all these registers. For eachregister, there is one dedicated enable conductor that extends from theI2C interface logic to an enable input lead of that register.

To write an 8-bit value into a particular register, the I2C interfaceplaces the data to be written onto the DATA conductors 81. Because theaccess is a write, the voltage on the R/W conductor 82 is driven to adigital logic low level. The enable conductors to all the registers aredriven to be disabled (digital logic low), except for the one desiredregister that is to be written. The enable conductor to that register isdriven with a digital logic high signal. After these signals are set up,the strobe signal on conductor 84 is pulsed high to clock the data intothe enabled register. The 8-bit value stored in a particular registercan be read by the I2C interface in similar fashion except that the I2Cinterface does not drive data out on the DATA conductors, but rather theI2C is setup to read in data from the DATA conductors. In addition, thedigital logic value driven onto the R/W conductor is a digital logichigh value. When the data bus conductors are set up this way, a pulsingof the strobe signal causes the enabled register to output its 8-bitvalue onto the 8-bit DATA bus, so that the 8-bit value will then belatched into the I2C interface logic. In this way, the I2C interface canread from, and can write to, any selected one of the registers on theintegrated circuit.

FIG. 3 is a more detailed diagram of the current switch circuit 57 ofFIG. 2. The current switch circuit 57 includes an 8-bit register 89 thatis accessible via the I2C interface and digital register control anddigital state machine block 58 of FIG. 2. Three of the bits are aVIN_REF_OV[2:0] value. This 3-bit value is supplied onto the selectinputs of the 8:1 analog multiplexer 90. The multiplexer determines themagnitude of a voltage VIN_OV_REF that is supplied onto an input lead ofcomparator 91. The voltage on the other input lead of comparator 91 is avoltage-divided version of the VIN voltage on the terminal 35.Accordingly, the 3-bit value is a programmable value that the voltage onthe VIN terminal, above which the digital signal OV is asserted toindicate an overvoltage condition. The register 89 also includes a bitthat is called an “override” bit. Setting this bit to a digital logichigh forces the current switch circuit 57 to open (to disconnect and todecouple the VIN terminal from the VOUT terminal). The through-currentflowing through the NFETs 92 and 93 is sensed and mirrored, and themirrored smaller current is then converted into a SC voltage signal bypassing the mirrored current across the RESET resistor 19. The voltagesignal SC is directly proportional to the magnitude of thethrough-current passing through the NFETs. If there is a supply voltagepresent on either the VIN terminal or on the STR terminal, then the LDO94 outputs a supply voltage VDD that is used throughout the integratedcircuit.

FIG. 4 is a more detailed diagram of the health monitor circuit 59 ofFIG. 2. The circuit of FIG. 4 includes eight compare-and-mask circuits95-102, a first mask register 103, a second mask register 104, an ORgate 105, a flag flip-flop 106, a pull-down transistor 107, asigma-delta ADC 108, a capacitor 109, an input analog multiplexercircuit 110, a decoder 111, a free-running on-chip RC relaxationoscillator 112, a channel monitor state machine 113, and a capacitorhealth check circuit 114. FIG. 5 is a more detailed diagram of one ofthe compare-and-mask circuits 95 of FIG. 4. FIG. 6 is a more detaileddiagram of the input analog multiplexer circuit 110 of FIG. 4. Thecapacitor health check circuit 114 includes a ten milliampere currentsink 115, a fifty milliampere current sink 116, a capacitor healthmonitor state machine 117, a DAC and compare circuit 118, an AND gate119, an SR latch 120, and a pull-down transistor 121. FIG. 7 is a moredetailed diagram of the DAC and compare circuit 118 of FIG. 4.

The structure of each of the compare-and-mask circuits is the same. Asshown in FIG. 5, compare-and-mask circuit 95 includes an 8-bit upperlimit register 122, an 8-bit upper limit FLASH register 123, an 8-bitlower limit register 124, an 8-bit lower limit FLASH register 125, an8-bit capture register 126, two 8-bit digital comparators 127 and 128,masking AND gate 129, masking AND gate 130, OR gate 131, an SR latch132, and a gate 133 that causes the SR latch to be reset. Each of theregisters 122-125 can be individually written with eight bits of datavia the I2C interface block 58 of FIG. 2. Each of the registers 122-126can be read via the I2C interface block 58 of FIG. 2. Also, the maskregisters 103 and 104 can be separately written with eight bits of datavia the I2C interface block 58 of FIG. 2. A counter in the channelmonitor state machine 113 of FIG. 4 counts up from 000 up to 111 andthen rolls over. Accordingly, the 3-bit SEL[2:0] values increment from000 up to 111, and then roll over back to 000, and this repeats. In thisway, the digital state machine sequentially selects the compare-and-maskcircuits, one at a time. The same selecting SEL[2:0] value is alsosupplied onto the select inputs of the input analog multiplexer circuit110. Consequently, the input analog multiplexer circuit 110 selectsanalog voltages on its analog data input leads, one at a time. In thecase of the first count SEL[2:0] being 000, the input analog multiplexercircuit is controlled to receive the analog voltage on the first datainput (CH1), and to supply a corresponding analog voltage onto the inputof the ADC 108. At the same time, the SEL[2:0] of 000 causes the firstcompare-and-mask circuit #1 95 to be selected. The ADC 108 performs ananalog-to-digital conversion, and outputs an 8-bit digital value ontoconductors 134. The conversion complete signal output by the ADC ontoconductor 135 causes the 8-bit digital value to be clocked into thecapture register (see FIG. 5) of the enabled first compare-and-maskcircuit 95. A comparison is made by the compare-and-mask circuit asdescribed below, and the new OR-ed result of all eight compare-and-maskcircuits is clocked into the flag flip-flop 106 at the conclusion of theone hundred millisecond ADC conversion operation. The digital statemachine 113 then increments the SEL[2:0] value, and this process isrepeated, except the second channel (CH2) is selected and the ADC resultis captured into the second compare-and-mask circuit 96. The voltagedividers 136-143 of the input analog multiplexer circuit 110 scale theinput voltage ranges of the various channels such that the range of themultiplexed analog signal at the input of the ADC is in the 0 volt to2.5 volt range. As shown in FIG. 6, the input analog multiplexer circuit110, in addition to the eight voltage dividers 136-143, includes ananalog multiplexer 159 made up of N-channel field effect transistors144-151 and a decoder 160. The digital state machine 113 controls theoverall circuit such that each successive input channel voltageconversion is captured into the capture register of each successivecompare-and-mask circuit.

Consider the case in which a given 8-bit ADC output value is clockedinto the capture register 126 of FIG. 5. Comparator 127 compares thecapture value to the value in the upper limit register 122. If thecapture value is greater than the upper limit value, then a digitallogic high is output by the comparator 127. Comparator 128 compares thecapture value to the value in the lower limit register 12. If thecapture value is smaller than the lower limit value, then a digitallogic high is output by the comparator 128. Each of the comparatoroutput can be separately and independently masked by setting ancorresponding bit in the appropriate mask registers. If both of the maskbits MASK1 and MASK2 of FIG. 5 are digital logic highs, however, thereis no masking. OR gate 131 outputs a digital logic high value if thecapture value is detected to be either lower than the lower limit valueor if it is higher than the upper limit value. If the output of the ORgate 131 is a digital logic high, then the SR latch 132 is set.Otherwise it is not set. The setting of the SR latch 132 indicates acondition of the capture value being out of the range between the upperlimit value and the lower limit value. As seen in the circuit diagram ofFIG. 4, the outputs of all of the eight SR latches of the eightcompare-and-mask circuits are ORed by OR gate 105, and if one or more ofthe ST latches are set, then the flag flip-flop 106 is set at the end ofthe conversion cycle. The setting of the flag flip-flop 106 causes thepull-down transistor 107 to be turned on, which in turns causes theinterrupt signal 78 (see FIG. 1) to be asserted, which in turninterrupts the microcontroller 34. The microcontroller 34 responds tothe interrupt by reading one or more registers within the power lossprotection integrated circuit 6 via the I2C block 58. In the presentexample, the microcontroller causes the I2C block 58 to read the 8-bitcapture value in register 126. The 8-bit value is returned to the I2Cblock 58 via the DATA lines 81, and then the 8-bit value is communicatedback across the I2C bus 80 to the microcontroller 34. The gate 133within the compare-and-mask circuit that was read detects that itscapture register is being read, and this causes the gate 133 to pulse adigital logic high pulse onto the reset input of the SR latch 132. Thisclears (resets) the latch. One of the registers (not shown) of theintegrated circuit is a read-only register consisting of the eight SRlatch values, considered together as an 8-bit value. The microcontroller34 can read this one “register” and thereby obtain the values of theeight SR latches. From this information, the microcontroller 34 candetermined which one of the eight compare-and-mask circuits it was thatcaused the fault. From this information, the microcontroller 34 can readthe capture register of that compare-and-mask circuit, thereby obtainingthe 8-bit capture value and at the same time resetting the SR latch.

Note that the undervoltage signal UV and the high current signal HC aresupplied to inputs leads of the OR gate 105 of FIG. 4. An undervoltagecondition or a high current condition will also cause the flag flip-flop106 to be set.

The capacitor health check circuit 114 has its own controlling statemachine 117. If the overall integrated circuit is not operating in thenormal mode as indicated by the digital signal NORMAL_MODE on conductor88, then the state machine 177 prevents the capacitor health checkcircuit 114 from performing an capacitor health check operation. If, onthe other hand, the overall integrated circuit is operating in thenormal mode as indicated by the digital signal NORMAL_MODE on conductor88, then at end of each four period the state machine 177 disables theconverter from operating in the boost mode by asserting the BOOST_DIS2signal, and enables the ten milliampere current sink 115. The currentdrain of this current sinking from the STR terminal causes the voltageon the external capacitor bank 11 to decrease. This current draincondition is maintained for a period of time determined by a 4-bitTSET[3:0] value in a register of the capacitor health check circuit. Atthe end of this time period, the state machine 117 disables the tenmilliampere current sink and simultaneously enables the fiftymilliampere current sink 116. The fifty milliamperes current sink 116then draws more current from the external capacitor bank 11 via terminalSTR. This condition persists for one tenth of the time period throughwhich the ten milliampere current source was enabled. If at any timeduring these two time periods the DAC and compare circuit 118 detectsthat the voltage on the STR terminal has fallen below a programmablevoltage, then the CAP_LOW digital signal is asserted and the SR latch120 is set. The setting of the SR latch 120 indicates a capacitor faultcondition. The pull-down transistor 121 is turned on, and the capacitorfault signal CF is asserted low. The programmable voltage is set, viathe I2C interface block 58, by setting the 4-bit value THR[3:0] in aprogrammable register (not shown) of the capacitor health check circuit114. The signal STR_FB coming into the DAC and compare circuit 118 ofFIG. 7 is a voltage-divided version of the voltage on the STR terminal44. If the voltage on the STR terminal is detected during these twoperiods as having gone below an STR undervoltage value STR_UV, then theDAC and compare circuit 118 of FIG. 7 asserts the STR_UV digital signaland this signal is communicated to the OR gate 105 of FIG. 4, which inturn causes the flag signal on flag terminal 50 to be asserted low. Ifthe voltage on the STR terminal is detected during these two periods ashaving gone above an STR overvoltage value STR_OV, then the DAC andcompare circuit 118 of FIG. 7 asserts the STR_OV digital signal and thissignal is communicated to the OR gate 105 of FIG. 4, which in turncauses the flag signal on flag terminal 50 to be asserted low. The valueof the SR latch 120 can be read via the I2C block 58 in the same waythat the SR latches of the compare-and-mask circuits 95-102 can be read.The reading of the SR larch 120 is detected by AND gate 119, whichcauses the SR latch 120 to be reset at the end of the reading. In FIG.7, the notation “110%” on a node of the voltage divider indicates thatthe voltage on that node is 110 percent of the voltage VDD, the notation“107%” on a node indicates that the voltage on that node is 107 percentof the voltage VDD, and so forth.

FIG. 8 is a state diagram that sets forth the operation of the capacitorhealth monitor state machine 117 of the capacitor health check circuit114. The signal name assignments in each state indicate the state towhich the state machine forced that signal when the state machine is inthat state. For example, in the IDL state, the notations “BOOST_DIS2=0”,“10MA=0” and “50MA=0” indicate that the digital signal BOOST_DIS2 isforced to be a digital logic low, and the ten milliampere current sinkis disabled, and the fifty milliampere current sink is disabled.

FIG. 9 is a table that indicates, for each 4-bit TSET[3:0] value, thecorresponding time period throughout which the ten milliampere currentsink is enabled.

FIG. 10 is a diagram that illustrates one capacitor health checkoperation as carried out by the capacitor health check circuit 114. TimeT1 is the end of a four minute period during which the overallintegrated circuit has been operating in the normal mode, and duringwhich the boost converter has been enabled. Due to the boost converterhaving been enabled this long, the capacitor bank 11 should be fullycharged to the voltage level set by the resistor RBSET 19. In thepresent example, this is 36 volts. At time T1, the capacitor healthmonitor state machine 117 disables the boost converter (by assertingBOOST_DIS2) and enables the ten milliampere current sink 115. The tenmilliampere current sink 115 remains enabled for the time perioddetermined by the TSET[3:0] value. This period of time ends at time T2.The state machine 117 then disables the ten milliampere current sink 115and enables the fifty milliampere current sink 116. This condition ofhaving the fifty milliampere current source enabled persists for onetenth of the TSET determined time. The diagram of FIG. 10 is not drawnto scale. This second period of time ends at time T3. The CF signal onterminal 51 may be asserted at any time between time T1 and time T3.Once it is asserted low, it remains asserted low due the setting of SRlatch 120. At time T3, the capacitor health check operation is over, andthe overall integrated circuit resumes operating in the normal mode.This means that the boost converter resumes operation. The capacitorbank 11 is therefore charged back up to the fully charged capacitorvoltage (in this case, 36 volts).

FIG. 11 is a diagram of the switching converter control circuit 61 ofFIG. 2. In the boost mode of operation, the controller 61 and theexternal components 20, 21, 23-27 operate as a boost converter. Therelatively low DC voltage on terminal 63 and external capacitor 21 isboosted up to a relative high voltage (for example, 36 volts DC), whichis driven out onto the STR terminal 44 so as to charge the capacitorbank 11 up to the relatively high voltage. In the buck mode ofoperation, the controller 61 and the external components 20, 21, 23-27operate as a buck converter. The relatively high voltage on capacitorbank 11 and the STR terminal 44 is bucked down to a relatively low DCvoltage. This relatively low voltage is driven onto the VOUT terminal 63and external capacitor 21. The magnitude of the relatively low voltage,which in the present example is 3.3 volts, is set by the externalresistors 25 and 26 (see FIG. 2). Advantageously, the same inductor 24is used in both the boost mode and in the buck mode. The switchingconverter control circuit 61 of FIG. 11 includes an 8-bit register 152.The microcontroller 34 can write an 8-bit value into this 8-bit register152 via the I2C bus 80 and the I2C interface block 58. The register 152includes an override bit 153. When this bit is set, the boost converteris forced on. The register 152 also includes override bit 154. When thisbit is set, the buck converter is forced on. If set, either of theseoverride bits takes precedence over the BOOST_DIS1 signal, the BUCK_ONsignal, the BOOST_DIS2 signal, and normal mode operation of the circuit61. It is an illegal condition that both these override bits 153 and 154to be set at the same time.

The magnitude of the relatively high voltage to which the converterboosts in the boost mode is user programmable, and is set by providingonly one external resistor RBESET 19 of the appropriate resistance. Themagnitude of the resistance of this one FBSET resistor correspondsdirectly to the magnitude of the relatively high voltage to which theSTR terminal is driven in the boost mode. The relatively high voltage towhich the STR terminal is driven in the boost mode is a gained-upversion of the voltage on the BSET terminal 45. If no external resistorRBSET is provided, then a twenty microampere current source 155 pullsthe voltage on BSET terminal 45 up to the LDO voltage level VDD of theVDD supply bus and node 156.

FIG. 12 is a more detailed diagram of the voltage reference circuit 62of FIG. 2. The voltage reference circuit 62 includes a bandgap voltagereference 157 and a comparator 158.

FIG. 13 is a block diagram of the I2C interface and digital registercontrol and digital state machine circuit 58 of FIG. 2. The block 58includes an I2C interface portion 161 and a digital state machineportion 162.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. An integrated circuit, comprising: a firstterminal; a second terminal; a current switch circuit that can receive acurrent of up to a maximum input current from the first terminal,wherein the current switch circuit can conduct a through-current fromthe first terminal, through the current switch circuit to the secondterminal with a resistance, and wherein the current switch circuitoutputs a current sense voltage whose magnitude is indicative of amagnitude of a current flowing through the current switch circuit; aninput analog multiplexer circuit having a first analog input lead, asecond analog input lead, at least one select input lead, and an analogoutput lead, wherein the first analog input lead is coupled to receivethe current sense voltage, and wherein the second analog input lead iscoupled to receive an input voltage; an Analog-to-Digital Converter(ADC) that is coupled to the output lead of the input analog multiplexercircuit, wherein the ADC is adapted to output a first digital valueindicative of a magnitude of the current flowing through the currentswitch circuit, and wherein the ADC is adapted to output a seconddigital value indicative of a magnitude of the input voltage; a firstcomparator circuit adapted to compare the first digital value to a firstlimit value and to output a first comparison signal indicative of thecomparison; a second comparator circuit adapted to compare the seconddigital value to a second limit value and to output a second comparisonsignal indicative of the comparison; and a third terminal adapted tooutput a flag signal, wherein the flag signal is indicative of acondition in which either the first comparison signal is asserted or thesecond comparison signal is asserted.
 2. The integrated circuit of claim1, further comprising: a first limit register adapted to store the firstlimit value; a second limit register adapted to store the second limitvalue; a first capture register adapted to store the first digitalvalue; and a second capture register adapted to store the second digitalvalue, wherein the maximum input current is at least two amperes,wherein the through-current is two amperes, wherein the resistance isless than one ohm, and wherein the integrated circuit does not include aprocessor that fetches, decodes or executes instructions.
 3. Theintegrated circuit of claim 1, wherein the first terminal is adapted toreceive the input voltage onto the integrated circuit, wherein the inputvoltage can have a magnitude of up to a maximum input voltage, whereinthe first terminal is coupled to the second analog input lead of theinput analog multiplexer circuit, wherein the integrated circuit furthercomprises digital logic circuitry adapted to control the input analogmultiplexer circuit by driving a digital logic signal onto the selectinput lead of the input analog multiplexer circuit, wherein the digitallogic circuitry is powered from a digital logic supply voltage, andwherein the digital logic supply voltage is a voltage smaller than themaximum input voltage.
 4. The integrated circuit of claim 1, furthercomprising: a fourth terminal, wherein the fourth terminal is adapted toreceive the input voltage onto the integrated circuit, and wherein thefourth terminal is coupled to the second analog input lead of the inputanalog multiplexer circuit.
 5. The integrated circuit of claim 1,wherein the current of which the current sense voltage is indicative isa current received by the current switch circuit from the firstterminal.
 6. The integrated circuit of claim 1, wherein the current ofwhich the current sense voltage is indicative is a current output by thecurrent switch circuit.
 7. The integrated circuit of claim 1, whereinthe current of which the current sense voltage is indicative is acurrent supplied by the current switch circuit to the second terminal.8. The integrated circuit of claim 2, further comprising: a third limitregister, wherein the first comparator circuit is also adapted tocompare the value stored in the first capture register to a value storedin the third limit register, and wherein the first comparison signal isasserted if either: 1) the value stored in the first capture register isgreater than the value stored in the first limit register, or 2) thevalue stored in the first capture register is smaller than the valuestored in the third limit register.
 9. The integrated circuit of claim1, wherein the input analog multiplexer circuit comprises: a firstresistor voltage divider circuit; a second resistor voltage dividercircuit; and an analog multiplexer having a first data input lead, asecond data input lead, a select input lead, and an output lead, whereinthe first data input lead is coupled to the first resistor voltagedivider circuit, and wherein the second data input lead is coupled tothe second resistor voltage divider circuit.
 10. The integrated circuitof claim 1, further comprising: a fourth terminal; and a switchingconverter control circuit adapted to output a direct current supplyvoltage onto the fourth terminal, wherein the fourth terminal is coupledto a third analog input lead of the input analog multiplexer circuit.11. The integrated circuit of claim 2, further comprising: a serial businterface circuit, wherein the serial bus interface circuit is operableto read the value stored in the first capture register and to cause thevalue to be output from the integrated circuit in serial fashion acrossa serial bus, and wherein the serial bus interface circuit is operableto read the value stored in the second capture register and to cause thevalue to be output from the integrated circuit in serial fashion acrossthe serial bus.
 12. The integrated circuit of claim 1, wherein the firstcomparator circuit comprises a first latch that outputs the firstcomparison signal, and wherein the second comparator circuit comprises asecond latch that outputs the second comparison signal.
 13. Anintegrated circuit, comprising: a first terminal; a second terminal; aneFuse circuit that can receive a current from the first terminal and canconduct the current to the second terminal; and an autonomous limitchecking circuit comprising an input analog multiplexer circuit, anAnalog-to-Digital Converter (ADC) coupled to an output lead of the inputanalog multiplexer circuit, a digital state machine that controls theinput analog multiplexer circuit, and a plurality of compare-and-maskcircuits, wherein each compare-and-mask circuit comprises a captureregister, a lower limit register, and an upper limit register, whereinthe compare-and-mask circuit is adapted to compare a digital valuestored in its capture register to a lower limit value stored in itslower limit register and to compare the digital value stored in itscapture register to an upper limit value stored in its upper limitregister, wherein the compare-and-mask circuit is adapted to assert adigital output signal indicative of a comparison performed by thecompare-and-mask circuit.
 14. The integrated circuit of claim 13,wherein a first analog input lead of the analog input multiplexercircuit is coupled to receive a voltage-divider divided down version ofa voltage present on the first terminal, and wherein a second analoginput lead of the analog input multiplexer circuit is coupled to receivea voltage whose magnitude is proportional to a current flowing throughthe eFuse circuit.
 15. The integrated circuit of claim 13, wherein thedigital state machine is powered from a digital supply voltage, andwherein the voltage present on the first terminal can be substantiallylarger than the digital supply voltage, wherein the integrated circuitfurther comprises a third terminal, and wherein a digital signal isasserted onto the third terminal if any of the compare-and-mask circuitsasserts its digital output signal.
 16. The integrated circuit of claim13, wherein the autonomous limit checking circuit further comprises amask register, and wherein a bit of the mask register being set causes acomparison performed by one of the compare-and-mask circuits to bemasked.
 17. The integrated circuit of claim 13, further comprising: aserial data bus interface, wherein the serial data bus interface isusable by external circuitry to read a selected capture register of theautonomous limit checking circuit, and wherein the input analogmultiplexer circuit comprises a plurality of resistor voltage dividers.18. The integrated circuit of claim 13, where each compare-and-maskcircuit has a bit that stores a value indicative of a comparisonperformed by the compare-and-mask circuit, wherein a reading of theselected capture register causes the bit of the compare-and-mask circuitthat includes the selected capture register to be reset, and wherein theintegrated circuit does not include a processor that fetches, decodes orexecutes any instructions.
 19. An integrated circuit, comprising: afirst terminal; a second terminal; an eFuse means for receiving acurrent from the first terminal and for conducting the current to thesecond terminal in a non-fault condition, and for opening in a faultcondition; and means for autonomously limit checking a plurality ofvoltages, for generating a digital value indicative of a magnitude ofeach such voltage, for each such voltage comparing the correspondingdigital value to an upper limit value and comparing the correspondingdigital value to a lower limit value, and for asserting a digital outputsignal indicative of a comparison pertaining to the correspondingdigital value.
 20. The integrated circuit of claim 19, furthercomprising: a third terminal, wherein a digital signal is asserted ontothe third terminal by the means for limit checking if any of themultiple such digital output signals is asserted; an upper limitregister adapted to store the upper limit value; and a lower limitregister adapted to store a lower limit value, wherein the integratedcircuit does not include a processor that fetches, decodes or executesany instructions.